I have a question I am not sure I know how to state properly so I will state a couple of assumed "givens" for a cruising boat. I have a good knowledge of DC and fixing and troubleshooting but the minutea of entire systems still seems to elude me. I want to learn that to prevent myself from having to buy batteries every 5 years. I want to only buy batteries every 15 years Cruising budgeting ya know.

The batteries on a cruising boat will never have a situation where there is no load at all on the batteries for any appreciable time. This is saying that even if off the boat the fridge will run on and off.

I am only speaking of tropical type cruising areas and/or summers in NE US. I don't care for the cold and have no plans for staying too long there.

At all times there will be some loads being taken off the batteries while the charging is taking place at the same time.

One of my assumptions is that the battery does not care what the charging source is as long as it is controlled properly when going to the battery. I mean that whatever the type of battery, the voltages and current are limited and correct.

I then don't know how the batteries and charging system react when the loads change from say a "thing" being used for 10 min and then turned off. Fridge is a good example. No current and batteries and charger "sitting idle". Then the "thing" turns on - I guess here is the question -

The charger then gives the batteries the current it was at "idle" plus the new draw, keeping everything equal and the battery topped up.

Another way to ask is if there is no charging source and the batteries are used to 50% which I think is the max recommended and then not loaded and charged up, then used again. I know this would not happen on a cruising boat because it would take too long to charge before a load was needed.

So what happens to all this - electrons, plates, electrolyte, wire, panels, wind gens, controllers, fridges etc - when stuff is being used?

Another way to ask is; the motor is running and the batteries are pretty much charged and the 100 amp alternator is putting out 12 amps and someone turns on the microwave and windlass. Does the alternator jump right in (I am not considering whether the alternator has a gradual start-up control) and just add the extra current?

Lots of questions I know but actually the same one.

Plus I am fully aware that different systems have different batteries, chargers and controllers etc. but I don't think that would change the answer to the fundamental question. Am I wrong?

In an earlier time, the now politically-incorrect answer would have been: "Now, sweetie, don't you worry your little head about it!"

Yes, smart chargers and regulators on alternators and other charging sources (solar, generators, wind, etc.) will automatically adjust for whatever load is put on the system.

Within reason, they will fully compensate so that the batteries will hardly know there's a change in load.

You're very right that on a cruising boat there's always some sort of load on the system -- well, almost always unless you take very specific steps. If the total charging current is enough to overcome whatever loads are on the system, little or no current will be taken from the batteries. If the total charging current is less than the load current, the difference (charging minus load) will come from the batteries. Smart chargers and regulators take care of all this for you.

You're very unlikely to get 15 years out of your batteries on a cruising boat. The best chance of doing that is to purchase high-end batteries, like the Rolls/Surette. But, of course, they cost a lot more than other batteries of comparable capacity. Whether they're cheaper in the long run is debatable, though some authorities believe they may be.

Some of the new battery technologies may achieve that kind of longevity...we'll have to wait and see. And, of course, it will depend also on how much they cost. Right now, for example, the Firefly technology (a Caterpiller spinoff company) looks great, and it's not expensive to produce. But we'll have to wait and see if the company can resist charging an arm and a leg for their products when they finally come to the general marketplace next year.

A simple way to think of it is to use the old "water" analogy. Water pressure being analogous to voltage, and water flow being analogous to amperage.

If you have a water pump (or are hooked up to dockside water via a pressure regulator fitting), the pump will kick on when it senses the water pressure is below some set threshold that the pressure switch senses - say 3 psi. Once the pressure is up to the cutoff limit - say 6.5 psi, the pump will stop.

So if you turn on one faucet, the pump won't turn on unless the pressure drops to 3 psi or lower. Once it does, the pump will kick in. In this water example, your accumulator (pressure vessel) acts like a battery in a DC electrical system. It stores energy in the form of compressed air. When you open a faucet, the pressure forces water through the plumbing system.

Now, compare that to our electrical system. The major conceptual difference between the two (and it's minor) is that with electricity, the losses are higher, and are usually dissipated as heat. And since a battery will self-discharge over time, you have to maintain a "float voltage" on them to keep them fully charged (pressurized). The analogous thing in a boat's water system is water leakages.

Older water pump technology was basically on/off, so the accumulator tank handles things until the pump kicks in. Newer variable speed pumps can now start up at a low level and "share" the load with any accumulator tanks in the system.

Now back to electricity. When you turn on a deck light, current starts flowing from the battery bank thru the wiring to the light. In this case, the voltage drop (water pressure drop) is slight, but it's there. Think of a modern battery charger as a variable speed water pump - it can sense the voltage level, and put out current to keep the voltage level up. If you start up a big load system like start the engine or use the windlass, then you're usually drawing more current from the batteries than the charger can match, so you'll have to keep running the charger for quite a while to replenish the amps that were drawn down from the batteries.

Like Bill says, it's unlikely with today's technology that you'll get batteries to last 15 years. With water pumps, basically the only wear and tear is from impurities in the water which cause build up of scale over time, and some wear and tear on o-rings, seals, diaphragms and the like. With batteries, there's a chemical reaction every time the batteries are discharged, and another every time they're charged. And, like in life, nothing's free. No chemical reaction is 100%, so there are losses over time. Sure, you can equalize your batteries, or even pulse-charge them in an attempt to crack sulfate crystals, but you never reach 100%, so eventually the batteries will be no longer usable.

Wanting 15 years? Typical might be more like 7 to 8 and then only if you try hard and do things properly all the time. With improper charging you can trash them in 9 months. The battery guy I use says he had a customer get 10 years from some Trojan golf cart batteries. He was sure because of the battery cases he could tell the date of manufacture. He also said 7 years is what they normally last.

If you cruise full time the cycles catch you sooner rather than later.

What I am trying to get a grasp of is the effects of constant flow through the batteries as a cruiser would see. On a molecular level and circuit switching sort of view - probably too much for me to hold in my mind though.

This flow fluctuates as does the charging. Not like my Jon boat that sits for weeks after a full charge and then runs the bait bucket bubbler.

PS: every battery I have ever had has outlasted others and the expected lifespan so I guess I am OK. It is just that I always want to know EVERYTHING even though I cannot grasp/comprehend a lot of it.